Method and continuous production line of plastic tubes with bi-axial drawing, and resulting plastic tube

ABSTRACT

The invention concerns a continuous manufacturing method which consists in producing a blank (E) by extrusion, bringing said blank to a temperature for molecular orientation, passing said blank around a radial expansion mandrel ( 12 ); gauging and cooling while subjecting the blank to axial traction. The radial expansion produced on the mandrel ( 12 ) is partial so that the internal diameter (B) of the blank (E), when it leaves the mandrel, is less than the nominal internal diameter (D) of the finished tube; performing an additional radial expansion of the blank, up to its nominal diameter, by internal fluid pressure. In permanent operating conditions, the inner surface of the blank (E) is no longer in contact with the mandrel ( 12 ).

The invention relates to a process for the continuous manufacture ofplastic pipes with biaxial drawing, in which a tube blank is formed byextrusion and then subjected to combined axial pulling and radialexpansion.

FR-A-2 753 648 relates to a process of this type, especially forrelatively large diameter pipes, in particular for diameters greaterthan 150 mm. This process, which is satisfactory, is however a batchprocess. In certain cases, especially for smaller pipe diameters, it isdesirable for the process to be continuous so as to further improve theproductivity and to reduce the investment cost per unit produced.

Continuous manufacture of a pipe with biaxial drawing poses a problem asregards performing the radial expansion of the pipe in one region of themanufacturing line. Furthermore, the startup phase of the process istricky.

FR-A-2 207 793 discloses a process for the continuous manufacture ofplastic pipes with biaxial drawing, in which the radial expansion isperformed with the aid of a frustoconical mandrel. Means for pulling onthe pipe are provided downstream and upstream of this mandrel. The largebase of the frustoconical mandrel is extended by a cylinder of the samediameter, which penetrates a sizing device. The external diameter of thelarge base of the truncated cone and of the cylinder which extends itdetermine the inside diameter of the finished pipe.

WO 95/25626 relates to a continuous manufacturing process of the samekind as the one above, with a mandrel for the radial expansion, thediameter of the large base of the mandrel defining the inside diameterof the finished pipe. Means for pushing the tube blank are providedupstream of the mandrel, while a haul-off is provided downstream of themandrel.

The use of a mandrel simplifies the line startup operation, especiallyas regards the radial expansion. The tube blank matches thefrustoconical outer surface of the mandrel and thus undergoes arelatively well controlled expansion. However, the friction forces arerelatively high.

Rumanian patent No. 80960 shows a mandrel of increasing diameter havinga section in the form of a cross, the arms of which are provided attheir ends with castors. A plastic pipe undergoes a radial expansion onthis mandrel, followed by a radial contraction. The cross shape of thecross-section of the mandrel means that the external surface of themandrel is not continuous, which may produce marks on the pipe and causevariable and uncontrolled drawing around the same circumference.

WO 90/02644 relates to a process in which the radial expansion of thepipe is obtained by internal hydraulic pressure exerted in a chamberclosed upstream by a first plug corresponding to the inside diameter ofthe tube blank and, downstream of a sizing device, by another plug whichcan be inflated by the effect of pressure. This plug, when not inflated,makes it possible to start up the line with an initial diameter andthen, by inflating it, to reach the nominal inside diameter of thefinished pipe.

WO 97/06940 also uses an inflatable downstream plug and provides a meansof controlling the expansion of the pipe in response to variousparameters which indicate, directly or indirectly, a variation in therelative speed at which the material is delivered into the expansionregion and extracted from this region. The expansion is performed byhydraulic pressure in a chamber which is closed by the inflatable plugand a conventional upstream plug.

The use of an inflatable plug in contact with a plastic pipe at arelatively high temperature poses problems as regards the choice ofmaterial for this plug. This is because it is necessary for the materialto have sufficient elasticity to allow the plug to be inflated, whilebeing resistant to the temperature and to the friction which are imposedby the plastic pipe. Furthermore, to achieve satisfactory sealingwithout causing excessively high friction between the plug and theinternal surface of the pipe, it is difficult to adjust the internalpressure on the inflatable plug.

A first object of the invention is, above all, to provide a process forthe continuous manufacture of plastic pipes with biaxial drawing whichallows simple startup, by avoiding the problems posed by an inflatableplug, and which makes it possible to reduce the friction forces to whichthe pipe is subjected in the steady manufacturing state.

To achieve this object, according to a first arrangement of theinvention, the process for the continuous manufacture of plastic pipeswith biaxial drawing consists in producing a tube blank by extrusion, intaking this tube blank to the molecular orientation temperature, inmaking this tube blank pass around a radial expansion mandrel and insizing and cooling the tube blank while subjecting it to an axialpulling force, and is characterized in that:

-   -   the radial expansion performed on the mandrel is partial so that        the inside diameter of the tube blank, when it leaves the        mandrel, is smaller than the nominal inside diameter of the        finished pipe,    -   and a further radial expansion of the tube blank, up to its        nominal diameter, is performed by internal fluid pressure,    -   the whole being such that, in the steady operating state, the        internal surface of the tube blank is no longer in contact with        the mandrel.

Advantageously, a chamber closed upstream and downstream of the mandrelis produced with the aid of at least an upstream plug and a downstreamplug, each plug ensuring sealing, for example by comprising at least onedisk made of a material sufficiently flexible for it to be able to flexand straighten in order to ensure sealing.

Advantageously, the downstream sealing plug of greater diameter than theupstream plug is introduced thanks to the initial expansion on themandrel, thus allowing a sealed volume to be generated.

The pressurized fluid for the further expansion of the tube blank isadmitted between the two sealing plugs.

A second object of the invention, which may be taken into considerationindependently or in combination with the first object, is to provide aprocess for the continuous manufacture of plastic pipes with biaxialdrawing which produces a practically constant axial elongation of theplastic.

To achieve this other objective, according to a second arrangement ofthe invention, which may be implemented independently or in combinationwith the previous arrangement, the process for the continuousmanufacture of plastic pipes with biaxial drawing consists in producinga tube blank by extrusion, in taking this tube blank to the molecularorientation temperature, in radially expanding the tube blank, withsizing and cooling, while subjecting the tube blank to an axial pullingforce, this process being characterized in that the friction forceexerted on the tube blank and the pipe is detected and in that at leastone operating parameter is acted upon in order to keep this frictionforce approximately constant.

Such regulation produces a constant axial orientation (axialelongation), resulting in the finished pipe having a constant thicknessand constant physical properties.

The friction force may be detected by measuring the pulling forceexerted on the pipe. The operating parameter acted upon in order to keepthe friction force approximately constant may consist of the internalpressure in the pipe and/or of a partial vacuum created around the pipein a sizing device. According to another possibility, the friction forceis regulated by injecting a lubricating fluid, especially water, betweenthe pipe and the wall of the sizing device.

The above regulation may be combined with upstream regulation designedto keep the characteristics of the tube blank constant.

To do this, the characteristics of the tube blank, in particular thethickness and diameter of the tube blank, are measured and, in responseto the results of the measurements, the extruder is acted upon in orderto keep the characteristics of the tube blank constant.

According to the first or the second arrangement, the pulling force onthe pipe is advantageously exerted at a single point, downstream of asizing region, by at least one haul-off.

The invention also relates to a line for the continuous manufacture ofplastic pipes with biaxial drawing, for implementing the first processarrangement, comprising:

-   -   an extruder conventionally equipped for forming a tube blank;    -   at least one tank for raising the tube blank to the molecular        orientation temperature;    -   a device for the progressive radial expansion of the tube blank,        comprising an expansion mandrel kept stationary relative to the        extruder;    -   a sizing device and a device for cooling the pipe,    -   the manufacturing line being characterized in that the mandrel        has a maximum diameter smaller than the nominal inside diameter        of the pipe, in that a plug is located upstream of the mandrel,        in that at least one other plug is located downstream of the        mandrel, in order to produce a seal depending on the nominal        inside diameter of the pipe, and in that a pressurized-fluid        admission means is provided at a point located between the two        sealing plugs, in order to admit pressurized fluid and to expand        the pipe to its nominal inside diameter.

In such a manufacturing line, in the steady state, the pipe is no longerin contact, via its internal wall, with the mandrel.

Advantageously, a coaxial tube is fixed to the extruder and the mandrelis fixed to the end of this tube which has a channel emerging radiallyvia at least one opening, between the two sealing plugs, for injectingpressurized fluid.

Advantageously, the biaxial orientation part of the manufacturing lineincludes means for pulling on the pipe, particularly a haul-off, at asingle point located downstream of the sizing device and of the coolingdevice.

The downstream plug may be located downstream of the cooling tanks andbe attached to the mandrel by a flexible linking element, especially acable.

As a variant, the downstream plug is located immediately behind themandrel, thereby reducing the size of the sealed volume.

The mandrel has an axisymmetric, for example frustoconical, overallshape, the large base of which faces the opposite side from theextruder; advantageously, the mandrel is provided, on its surface, withrolling members distributed angularly and axially so as to avoid markingthe internal surface of the pipe and to reduce the friction forces.

The rolling members preferably consist of cylindrical or sphericalrollers mounted so as to rotate.

Advantageously, the final tank for raising the temperature and/or thesizing device are mounted so as to be able to be moved one with respectto the other, in axial translation, especially to change theirseparation, and make it easier to start up the manufacture.

The invention also relates to a line for the continuous manufacture ofplastic pipes with biaxial drawing which, independently of the abovefeatures or in combination with these features, comprises:

-   -   an extruder conventionally equipped for forming a tube blank;    -   at least one tank for raising the tube blank to the molecular        orientation temperature;    -   a device for radially expanding the tube blank by internal fluid        pressure;    -   a sizing device and a device for cooling the pipe;    -   means for pulling the tube blank axially; this manufacturing        line being characterized in that it includes means for detecting        the friction force exerted on the tube blank and the pipe, and        means for acting upon at least one operating parameter of the        manufacturing line in order to keep this friction force        approximately constant.

Advantageously, the means for pulling on the tube are located at asingle point, downstream of a sizing region, and comprise at least onehaul-off.

The friction force may be detected by means for measuring the pullingforce exerted on the pipe.

Advantageously, means for measuring the characteristics of the tubeblank, in particular the thickness and diameter of the tube blank, areprovided and means for acting upon the extruder in response to theresults of the measurements, in order to keep the characteristics of thetube blank constant.

The invention also relates to a biaxially drawn plastic pipe obtained bythe process and/or the manufacturing line described above.

The invention consists, apart from the arrangements presented above, ofa number of other arrangements which will be explained in more detailbelow in regard to illustrative examples described in detail withreference to the drawings appended hereto, but which are in no waylimiting.

FIG. 1 of these drawings is a schematic axial section, with parts on theoutside and parts removed, of a the manufacturing line as claimed in theinvention, at the start of production.

FIG. 2 shows schematically part of the line after the expansion mandrelhas been put into place at the inlet of the sizing device.

FIG. 3 shows, in a manner similar to FIG. 2, the manufacturing line atthe start of injecting the pressurized fluid.

FIG. 4 shows, in a manner similar to FIG. 3, the manufacturing lineafter a plug has been placed downstream of the haul-off.

FIG. 5 shows, in a manner similar to FIG. 4, the manufacturing line whenthe production steady state has been established.

FIG. 6 shows, in a manner similar to FIG. 5, an alternative.

FIG. 7 is a side view on a larger scale of the radial expansion mandrel.

FIG. 8 is a view, on the line VIII—VIII in FIG. 7, of one element of themandrel.

FIG. 9 is a top view of the mandrel element of FIG. 8.

Finally,

FIG. 10 is a side view of an illustrative example of an upstream plug.

In the drawings, especially FIG. 1, may be seen a manufacturing line 1employing the process of the invention. The production is carried outcontinuously from the upstream end (to the left in FIG. 1) to thedownstream end (toward the right in FIG. 1).

The manufacturing line comprises, shown schematically, an extruder 2 fedwith thermoplastic, especially PVC, from a hopper 3 shown schematically(dry-blend feed). The extruder 2 is provided at its front end, on theopposite side from the hopper 3, with tooling 4 for extruding a pipetube blank E. The tooling 4 includes one or more lateral inlets 5 foradmitting and/or discharging pressurized air, or more generally apressurized fluid (gas or liquid). Furthermore, a sizing die (not shown)is conventionally provided.

A coaxial metal tube 6 is fixed to the extruder 2 and extends toward theright in FIG. 1. This tube 6 has, toward its closed axial end, at leastone radial opening 7. When pressurized air is admitted via the inlet 5,this air is directed by channels (not shown) from the tooling 4 into thetube 6, to leave via the opening 7.

A pipe tube blank E leaves the extruder 2 at a relatively hightemperature of around 150° C. or more and passes through one or morecooling tanks 8, generally containing water, which are thermallyregulated for example to 10-20° C. in order to substantially lower thetemperature of the tube blank E. A conventional haul-off (not shown) isgenerally provided downstream of the cooling tanks 8. This haul-off,which acts upon a tube blank when still hot, and therefore deformable,is not actually used to transfer thrust forces but only to decouple theextrusion, and biaxial drawing functions, by damping the perturbationsand interaction of one on the other.

The biaxial orientation part of the tube blank starts downstream of thehaul-off (not shown) of the extrusion part. This biaxial orientationpart comprises a thermally regulated tank 9, for cooling or heating, inorder to raise the tube blank E to a temperature lying within the rangeof molecular orientation temperatures. For PVC, this temperature lieswithin a range from 90° C. to 110° C.

Fixed to the outlet of the tank 9 is a ring 10 for externally pressingthe tube blank onto an internal plug 11 upstream, shown in FIGS. 2 to 6.This internal plug 11 is located upstream of orifice 7.

As may be seen in FIG. 10, the internal plug 11 may comprise, from theupstream end to the downstream end, a succession of sealing rings,especially two relatively flexible rings 11 a, 11 b, for example made ofan elastomer, the periphery of which is formed by a frustoconical lipwhose large base faces downstream. Several disks 11 c, 11 d, 11 e, 11 f,for example made of polytetrafluoroethylene (PTFE), spaced apart,parallel to one another and perpendicular to the axis of the tube blank,are provided downstream of these rings 11 a, 11 b. The outside diameterof the rings and of the disks is at least equal to the inside diameterof the tube blank E.

The succession of rings and disks 11 a-11 f forms a stack held by ametal core 11 g locked together, so as to be able to be adjusted alongthe axial direction, by nuts 11 h, 11 i screwed on to the tube 6consisting of a hollow threaded rod.

The Example of the plug 11 described in detail above is not limiting,any possible equivalent sealing means being suitable.

A radial expansion mandrel 12, which can be seen in FIG. 2, is fixed,under conditions explained below, to that end of the tube 6 on theopposite side from the extruder 2. The mandrel 12 has an axisymmetricoverall shape and consists, for example, of a truncated cone. The largebase 12 a of the mandrel faces downstream. The diameter B of the largebase 12 a is smaller than the nominal inside diameter D (FIG. 4) of thefinished pipe T.

Preferably, the diameter B is at least 10% smaller than the diameter D.

As illustrated in FIG. 7, the mandrel 12 is advantageously provided onits surface with cylindrical or spherical rollers 13, mounted so as torotate on spindles (not visible) carried by the mandrel 12. The rollers13 are partly housed in cavities on the surface of the mandrel andproject from this surface. The axes of rotation of the rollers 13 lie inseveral parallel planes, perpendicular to the axis of the mandrel 12.The rollers 13, lying in the same plane, are uniformly distributedaround the axis of the mandrel. The rollers lying in successive planesare offset in a staggered fashion so as to prevent any generatrix of themandrel 12 being devoid of a roller.

Advantageously, the mandrel 12 consists of a frustoconical stack ofdisks 14 having diameters which increase from the upstream end to thedownstream end. Each disk has rollers 13 uniformly distributed on itsperiphery in the manner of a crown. The disks 14 have a central opening15 with one or more recesses 16 for mounting on a shaft 17, of conjugatesection, so as to be prevented from rotating. An axial stop 18 isprovided on the shaft 17 in order to retain the smallest diameter disk.On the opposite side, the shaft 17 has a thread 19 allowing the stack ofdisks 14 to be locked with the aid of a nut engaging with this thread.

The mandrel 12 may thus be adjusted to various inside diameters of tubeblanks by adding or removing disks 14 of appropriate diameters.

The cylindrical or spherical rotating rollers 13 allow the friction ofthe tube blank on the mandrel to be considerably reduced and preventmarks on the internal surface of the pipe.

A coaxial plug 20 is fixed to the mandrel 12 on the same side as thelarge base 12 a, on the opposite side from the tube 6. This plug 20 is,for example, formed by at least one, and preferably several, disks 21spaced apart, parallel to one another, which are made of a plastichaving a low coefficient of friction, such as polytetrafluoroethylene(PTFE) or a thermoplastic elastomer (TPE), which may or may not bereinforced. The disks 21 are made of a material sufficiently flexible tobe able to flex and straighten in order to ensure sealing. The plug 20is designed to ensure sealing at least along a diameter equal to thediameter B of the large base of the truncated cone 12. Otherexplanations on this subject will be given with regard to the variant inFIG. 6.

A flexible linking member 22, for example a cable, is furthermoreattached to the center of the mandrel 12 on the opposite side from thetube 6. The plug 20 may be installed on that part of the cable 22 closeto the mandrel 12.

Downstream of the mandrel 12, the tube blank enters a sizing device 23(shown schematically). This sizing device 23 is placed in a water-sprayvacuum cooling tank 24. The spraying of water on the external surface ofthe plastic pipe leaving the sizing device 23 allows the material tosolidify. On leaving the tank 24, the pipe T in the finished statepasses into a haul-off 25 which exerts the necessary pulling force onthe entire line in order to haul the pipe and the tube blank. Thehaul-off 25 conventionally consists of two caterpillar tracks 26, 27bearing against opposed regions of the pipe in order to haul it from theleft to the right in FIG. 1. Each caterpillar track has a flexibleelement 28 forming an endless loop wrapped around wheels 29, at leastone of which is driven, these wheels 29 being mounted so as to rotateabout spindles 30 supported by a stand 31 fixed to the ground.

The pulling force exerted by the haul-off 25 is detected by a sensor 32(FIG. 1), for example at the point where a spindle 30 is mounted on thestand 31.

The output of the sensor 32 (FIG. 1) is connected to a unit 33, allowingthe degree of vacuum in the tank 24 to be adjusted in response to theoutput signal from the sensor 32, and to a unit 34 allowing the pressureof the air delivered by the orifice 7 to be adjusted.

The unit 33 may, if necessary, cause water to be injected with the aimof reducing the friction between the pipe and sizing device 23 if theforce measured by the sensor 32 is too high.

A sensor j is advantageously provided at the exit of the extruder 2 inorder to measure the diameter of the tube blank. Several other sensorsh, for example ultrasonic sensors, are distributed around the peripheryof the tube blank, close to the exit of the extruder 2, in order tomeasure the thickness of this tube blank. The sensors j and h areconnected to a control unit K suitable for acting upon the operatingparameters of the extruder 2 in order to keep the dimensions of the tubeblank, at the exit of the extruder 2, as constant as possible accordingto the values measured by the sensors j and h.

The tank 24 and the sizing device 23 are mounted so as to be adjustablein the axial direction, as is the tank 9. These elements are mounted soas to move in translation, for example on rails fixed to the ground (notshown in the drawing), with means for blocking in a desired position.

The distance L between the downstream face of the tank 9 and theupstream face of the tank 24 is thus adjustable.

In the steady operating state, a downstream plug 35 (FIG. 4) fastened tothe cable 22 bears in a sealed manner against the internal surface ofthe pipe T. The plug 35 may consist of disks 36 which are spaced apart,parallel to one another and perpendicular to the axis of the tube andare made of a material such as PTFE or a thermoplastic elastomer (TPE),which may or may not be reinforced. The plug 35 is designed to providesealing along the inside diameter D of the finished pipe.

From the foregoing description, it is apparent that the manufacturingline is composed of two different parts, with the minimum ofinteractions:

-   -   a tube blank extrusion part consisting of a conventional        extrusion line (extruder 2, tooling 4, sizing die, not shown,        cooling tanks 8, haul-off, not shown);    -   a tube blank biaxial orientation part with tanks 9 for bringing        it to temperature, expansion device (12; 6, 7, 11, 20, 35, 20        a), sizing device 23, cooling device 24, haul-off 25 and saw        (not shown).

This being the case, the operation of the manufacturing line, employingthe process of the invention, is as follows.

The line startup is illustrated by FIGS. 1 to 4.

According to FIG. 1, a tube blank E leaves the extruder 2, in arelatively pasty state, and travels right down the line. To ensure thistravel down the line, an auxiliary pulling tube may be used which isintroduced from the downstream end, upstream into the line until comingclose to the exit of the extruder; the tube blank is attached by anysuitable means to this pulling tube which, hauled by the haul-off 25,hauls the tube blank itself through the tanks 8, 9 and 24.

The radial expansion mandrel 12 is not yet installed (FIG. 1) and thereis no pressurized air injection via the inlet 5 and the outlet orifice7. The tank 9 is set back upstream, while the tank 24 is advanceddownstream so that the distance L is the maximum.

The mandrel 12 and the plug 20 are then fitted. To do this, a slit ismade along a generatrix in the wall of the tube blank in a region lyingbetween the tank 9 and the tank 24. The edges of the slit are movedapart in order to fit, downstream of the orifice 7, the plug 11 asillustrated in FIG. 2. The mandrel 12 is then fixed to the end of thetube 6, for example by screwing, and the cable 22 is introduced alongthe entire length of the tube blank so as to be able to recover the endof this cable downstream of the tank or tanks 24 for cooling thebiaxially oriented pipe, optionally cutting off, using a saw (notshown), a downstream part of the tube blank.

With material continuing to arrive, the slit made in the tube blank, forfitting the plug 11 and the mandrel 12, moves downstream and disappears.At this moment, the tube blank undergoes its first expansion in contactwith the mandrel 12 (FIG. 2). There is still no injection of pressurizedair via the orifice 7.

When the tube blank has been correctly closed off downstream of themandrel 12, the tank 9 is moved downstream, as illustrated by an arrowin FIG. 3, so that the ring 10 is placed around the plug 11. Air is theninjected via the inlet 5 and the orifice 7, with a gradual rise in thepressure so that the tube blank separates slightly from the mandrel 12,as illustrated in FIG. 3.

With increasing air pressure, the tube blank moves completely away fromthe mandrel 12, as illustrated in FIG. 4, and comes into contact withthe sizing device 23. The inside diameter of the pipe then reaches itsnominal value D greater than the diameter B of the large base of themandrel 12.

The downstream plug 35 is fitted to the end of the cable downstream ofthe cooling tank or tanks 24 and thus provides further sealing.

As a variant, it defines with the downstream plug 20 a chamber in whichair pressure provides, after the sizing device, the additional coolingof the formed pipe.

When the steady state has been established, the tank 24 is moved towardthe tank 9, as illustrated by an arrow in FIG. 5, in order to reduce thespace between these two tanks and increase the distance between theinternal surface of the tube blank and the mandrel 12.

Preferably, a pulling force is exerted on the tube only downstream ofthe tank 24. The value of the pulling force measured by the sensor 32(FIG. 1) constitutes a measurement of the friction of the tube blank andof the pipe in the line, particularly in the sizing device 23. Theregulating effected on the basis of this force measurement by the sensor32 makes it possible to work with a pulling force, and therefore afriction force, which is approximately constant, thereby correspondingto an approximately constant axial orientation (axial elongation) of thematerial of the pipe T.

The friction force is kept approximately constant by the unit 34(FIG. 1) which acts upon the internal pressure, in the tube blank and inthe pipe, according to the value detected by the sensor 32. It is alsopossible to act upon the degree of vacuum in the tank 24, around thepipe, or by injecting lubricating water between the pipe and theinternal wall of the sizing device 23.

This regulation is effective if the dimensional characteristics of thetube blank are constant. To improve the manufacture, provision may bemade to also regulate the tube blank leaving the extruder 2 with the aidof sensors j and h (FIG. 1) which, via the values detected, allow theunit K to change the operating conditions of the extruder 2 in order tokeep the dimensional characteristics of the tube blank approximatelyconstant.

FIG. 6 illustrates an alternative embodiment in which the downstreamplugs 20 and 35 of the previous embodiment now form only a single plug20 a located immediately downstream of the mandrel 12. This plug 20 aalso preferably consists of parallel disks 21 a, perpendicular to theaxis of the tube. The disks must be flexible enough to be able to flexand, in a first step ensure sealing along the diameter B correspondingto the large base of the mandrel 12. The disks 21 a of the plug 20 amust be able, by dint of their elasticity, to straighten and then ensuresealing along the nominal inside diameter D of the pipe. The approachshown in FIG. 6 has the advantage that

the closed chamber lying between the upstream plug 11 and the downstreamplug 21 a has a smaller volume. This results in lower inertia as regardspressure variations and temperature variations which may be controlled.Since the volume of air stored is smaller, should a leak or an incidentoccur, the risks are also reduced.

In the foregoing description, the fluid used to generate the internalpressure in the tube blank is air. Of course, this fluid could be aliquid, in particular hot water.

1. A process for continuously manufacturing plastic pipes with biaxialdrawing, comprising: forming a tube blank by extrusion, the tube blankbeing taken to the molecular orientation temperature, made to passaround a radial expansion mandrel member, and sized and cooled whilebeing subjected to an axial pulling force, wherein the radial expansionperformed on the mandrel member is partial so that the inside diameterof the tube blank, when it leaves the mandrel member, is smaller thanthe nominal inside diameter of the finished pipe; and performing afurther radial expansion of the tube blank, up to its nominal diameter,by an internal fluid pressure; whereby, in the steady operating state,the internal surface of the tube blank is no longer in contact with themandrel member.
 2. The process as claimed in claim 1, wherein a chamberclosed upstream and downstream of the mandrel member is formed with theaid of at least one upstream plug and at least one downstream plug, eachplug comprising at least one disk made of a material sufficientlyflexible for it to be able to flex and straighten to ensure the sealingof the chamber.
 3. The process as claimed in claim 2, wherein thedownstream plug has a greater diameter than that of the upstream plug,the downstream sealing plug being introduced subsequent to the initialexpansion on the mandrel member, allowing a sealed volume to begenerated.
 4. The process as claimed in claim 2 further comprisingadmitting a pressurized fluid between the two sealing plugs for furtherexpansion of the tube blank.
 5. The process as claimed in claim 1further comprising detecting a friction force exerted on the tube blankand the pipe, wherein at least one operating parameter is acted upon inorder to keep the friction force approximately constant.
 6. The processas claimed in claim 5, wherein the friction force is detected bymeasuring the pulling force exerted on the pipe.
 7. The process asclaimed in claim 5, wherein the operating parameter comprises theinternal pressure in the pipe and/or of a partial vacuum created aroundthe pipe in a sizing device.
 8. The process as claimed in claim 5,wherein characteristics of the tube blank are measured and, in responseto the results of the measurements, the extruder is acted upon in orderto keep the characteristics of the tube blank constant.
 9. The processas claimed claim 1, wherein the pulling force on the pipe is exerted ata single point located downstream of a sizing device and a coolingdevice.
 10. A plastic pipe obtained by the process as claimed inclaim
 1. 11. A manufacturing line for the continuous manufacture ofplastic pipes with biaxial drawing, comprising: an extruder memberadapted to form a tube blank; at least one tank member for heating thetube blank to the molecular orientation temperature; an expansion devicefor progressively radially expanding the tube blank, the expansiondevice comprising an expansion mandrel member kept stationary relativeto the extruder member, the maximum diameter of the mandrel member beingsmaller than the nominal inside diameter of the finished pipe; a firstplug member located upstream of the mandrel member; a second plug memberlocated downstream of the mandrel member for producing a seal dependingon the nominal inside diameter of the pipe; and a pressurized-fluidadmission means located between the upstream plug and the downstreamplug for admitting a pressurized fluid and for expanding the pipe to itsnominal inside diameter.
 12. The manufacturing line as claimed in claim11, wherein, in the steady state, the internal wall of the pipe is awayfrom the mandrel member.
 13. The manufacturing line as claimed in claim11 further comprising a coaxial tube fixed to the extruder, wherein themandrel member is fixed to the end of the coaxial tube that has achannel emerging radially via at least one opening, upstream of themandrel member, and downstream of the upstream plug for injectingpressurized fluid.
 14. The manufacturing line as claimed in claim 11further comprising means for pulling the pipe located downstream of asizing device and of a cooling device.
 15. The manufacturing line asclaimed in claim 14, wherein the downstream plug is located downstreamof the pulling means and attached to the mandrel member by a flexiblelinking element.
 16. The manufacturing line as claimed in claim 14further comprising means for measuring the pulling force exerted on thepipe.
 17. The manufacturing line as claimed in claim 11, wherein thedownstream plug is located immediately behind the mandrel member. 18.The manufacturing line as claimed in claim 11, wherein the mandrelmember comprises an axisymmetric shape, the large base of which facesthe opposite side from the extruder.
 19. The manufacturing line asclaimed in claim 18, wherein the mandrel member comprises afrustoconical overall shape, the large base of which faces the oppositeside from the extruder.
 20. The manufacturing line as claimed in claim18, wherein the mandrel member is provided on its surface with rollingmembers distributed angularly and axially so as to reduce the friction.21. The manufacturing line as claimed in claim 20, wherein the rollingmembers comprise cylindrical or spherical rollers mounted so as torotate.
 22. The manufacturing line as claimed in claim 11 furthercomprising a sizing device, wherein the tank for raising the temperatureand/or the sizing device are so mounted that they are capable of movingrelatively to each other, in axial translation, to change theirseparation.
 23. The manufacturing line as claimed in claim 11 furthercomprising means for detecting the friction force exerted on the tubeblank and the pipe, and means for acting upon at least one operatingparameter of the manufacturing line to keep the friction forceapproximately constant.
 24. The manufacturing line as claimed in claim11 further comprising means for measuring characteristics of the tubeblank, and means for acting upon the extruder in response to the resultsof the measurements to keep the characteristics of the tube blankconstant.
 25. The manufacturing line as claimed in claim 24 furthercomprising means for measuring the thickness or the diameter of the tubeblank.